When a firearm is discharged in the open atmosphere, the rapid release of pressurized propellant gasses through the muzzle creates a soundwave that travels through the air and to the ears of persons nearby. The sound travels by creating a compression wave through the air. This sound impulse can be reduced to a hearing-safe level by the use of a firearm noise suppressor, which typically includes a series of sealed chambers that temporality capture the high-pressure gas, allowing it to enter the atmosphere more slowly.
Because water is more dense than air, sound travels more than four times as fast through water than through air. Moreover, unlike air, water is nearly incompressible. When a firearm is discharged under water, the sound shockwave created is even more intense and travels even faster to the ears of people in the water. Discharging a firearm under water presents many challenges, including suppression of the sound shockwave to a level that is hearing safe and using the recoil energy to automatically extract a spent cartridge and load a new cartridge into the chamber without manually cycling the action or using a different, pre-loaded chamber.
Various underwater guns have been developed, many of which use a sealed barrel and/or flash and recoil suppressor, are capable of firing only a single shot without switching barrels/chambers, or that are effective only at contact range. One such device is shown in U.S. Pat. No. 3,300,888, which is a single shot device with a muzzle seal that is displaced or penetrated by the fired projectile. Another device “capable of being fired immediately after being subjected to moderate external water pressures” is described in U.S. Pat. No. 3,677,132, which shows a sound, flash, and recoil suppressor attachable to the threaded muzzle of a firearm barrel. This device uses a series of elastomeric wipes that are intended to reseal after penetration by a projectile.
U.S. Pat. No. 5,966,858 shows a muzzle brake and seal system that provides an evacuated chamber with internal baffles and a ball value at an exit port that is synchronized with the firing of a projectile. This system is intended to completely capture the propellant gases and any water that enters the capture chamber must be purged out between shots. U.S. Pat. No. 5,639,982 provides a method of firing a weapon underwater in which a blank cartridge is fired immediately before discharging a live round to purge water from the barrel and create a temporary bubble of gas at the muzzle. According to the method, once the repeated firing of live rounds has ended, another blank cartridge is used to again purge the barrel and create a bubble at the muzzle before firing another live round. Other devices use a series of pre-loaded chambers/barrels, like a revolver, that are selectively fired. In addition to other limitations, these devices would not be hearing-safe.
More recently, a water-filled sound and flash suppressor attached to the muzzle of a pistol has been used. This system temporarily captures and slows the propellant gases leaving the muzzle, forcing water inside the suppressor chambers to vent through radial ports. Although this device is effective at disrupting the sound shockwave produced by discharging the firearm, there may not be sufficient back pressure to cycle the slide or bolt, effectively making it a single shot system that must be manually cycled between rounds.